This invention relates in general to sealing systems for liquid storage tanks and, in particular, to a seal system for preserving the integrity of liquid contained in a storage container or tank having a roof freely floating on the contents contained within the storage tank.
More specifically, but without restriction to the particular use which is shown and described, this invention relates to a sealing system for a liquid storage tank having a roof freely floating on the contents contained within the tank and includes a sealing system which is adapted to function as a primary and secondary seal to prevent contamination of the stored liquid and to minimize or eliminate evaporation loss.
When storing liquids in large containers or storage tanks of the type having a roof freely floating on the contents contained in the tank, it is necessary in order to prevent contamination of the stored liquid by rain, snow, dust, dirt and the like to effect a seal between the floating roof and the inner walls of the tank shell to prevent these contaminants from entering into the tank and thereby contaminating the liquid contents. In addition, it is both desirable and necessary to prevent loss of the stored liquid through evaporation. Such evaporation losses due to vaporization are not only costly due to loss of the stored liquid, but are frequently required to be controlled due to governmental pollution agencies' regulations that require the storage tanks to meet specific emission control or vapor loss standards. Therefore, such storage tank sealing systems should be capable of preventing contaminant material from entering the storage tank and be effective to minimize or effectively eliminate evaporation losses.
In order to meet these preferred requirements, the sealing systems for storage tanks should conform to any distortions or irregularities in the inner walls of the storage tank shell. In addition, it is desirable that an adequate outward thrust be applied by the seal against the tank shell to minimize or eliminate gaps between the seal and the internal shell walls and to prevent shifting of the floating roof due to wind loads.
Many and various approaches have been taken in an attempt to provide such a sealing system, such as, for example, the structures disclosed in U.S. Pat. No. 3,589,549 and U.S. Pat. No. 3,373,891. However, with sealing systems utilizing both a primary and a secondary seal, in order to inspect the primary seal to insure that it is still effective and does not require repair, it is desirable to have easy access to the primary seal without requiring that the seal or associated structure be removed. In such sealing systems utilizing both a primary and a secondary seal, it is necessary that the secondary seal be readily positionable to allow inspection of the primary seal without requiring extensive labor or causing damage to the secondary seal when it is necessary to inspect the primary seal.
Another problem associated with many prior art sealing systems which utilize primary and secondary seals is that the secondary seal must be attached to the primary seal and, therefore, does not allow for easy inspection. Furthermore, in the event the primary seal becomes distorted or loses its sealing capabilities, so does the secondary seal. Such seal attachment also requires that the seal be removed from the tank if the tank shell structure itself ever needs repair because the burning or welding on the tank structure would destroy the seals.
In certain prior art the seals are made of a fabric which is susceptible to puncture and tearing. The metallic-shoe, such as disclosed in U.S. Pat. No. 3,373,891 was designed in an attempt to eliminate this problem. However, complicated and complex linkage must be utilized to maintain a sufficient sealing force against the inner wall of the tank shell to prevent the roof from moving due to wind loads. If such linkage malfunctions the seal is broken. Another problem created by a metallic-shoe seal is due to the capillary action between the shoe and the inner wall of the tank shell. Since the shoe is immersed in the liquid contained within the storage tank, a capillary effect results in the liquid contained within the storage tank rising upwardly between the inner wall of the tank shell and the metallic-shoe. This capillary effect results in the liquid rising above the effective seal area.